1. Field of Invention
The present invention relates to a fan and, in particular, to a centrifugal fan.
2. Related Art
Since the electronic industries have been in progressive in the recent years, the electronic products may include many heat-generating electronic components, such as chips, that generate more and more heat. In order to effectively dissipate the redundant heat, it is usually to dispose a heat sink on the surface of the electronic component and then to install a centrifugal fan on the heat sink. The centrifugal fan can generate cooling airflow to remove the heat from the heat sink, thereby dissipating the heat of the electronic components.
FIG. 1A is a top view of a conventional centrifugal fan 1, and FIG. 1B is a sectional view of the centrifugal fan 1 of FIG. 1A. As shown in FIGS. 1A and 1B, the centrifugal fan 1 includes an impeller 11 and a casing 12. The impeller 11 is disposed in the casing 12. The impeller 11 has a hub 111 connecting with the casing 12 and a blade 112 disposed surrounding the hub 111. The casing 12 includes a upper casing 121 and a lower casing 122, which are connected to each other. The impeller 11 is disposed between the upper casing 121 and the lower casing 122.
The surfaces of the upper casing 121 and the lower casing 122 are planar surfaces, and a gap exists between the blade 112, the upper casing 121 and the lower casing 122. Accordingly, when the pressure inside the fan increases to be larger than the external pressure, the air can flow through the gap and then exit the fan through the inlet openings of the upper casing 121 and lower casing 122 (see arrows). Thus, the air input quantity as well as the air output quantity may decrease, thereby affecting the entire heat-dissipation performance.
Therefore, it is an important subject to provide a centrifugal fan that can prevent the air reflow, which decreases the air input quantity, and effectively drive the air flowing toward the outlet opening so as to increase the air output quantity, thereby enhancing the heat-dissipation performance.